There has heretofore been a conventional information recording medium of the type in which the surface of a thermoplastic resin layer is charged by means of an electron beam in a vacuum to store information electric charge thereon and, after the charging, the thermoplastic resin layer is softened by heating to form on the surface thereof a frost image comprising dimple patterns corresponding to the quantity of electric energy stored thereon.
However, most conventional apparatuses designed to effect image formation in a vacuum are large in size and have difficulty in forming a frost image, depending on the type of thermoplastic resin used.
In the meantime, recording media that need no vacuum system are also known, for example, a thermoplastic recording medium that comprises a photoconductive layer and a thermoplastic resin layer, which are successively stacked on an electrode, and a thermoplastic recording medium comprising a single layer having both thermoplasticity and photoconductivity. There has also been a known method of recording information on such thermoplastic recording media, wherein, after a thermoplastic recording medium is subjected to initial overall charging by corona charging, for example, image exposure is effected, and the medium is fully charged again and then developed by heating or in a solvent vapor, thereby forming a frost image corresponding to the image exposure.
According to this developing method, a photoconductive member 10, which comprises an electrode 10b and a thermo-plastic resin layer 10a that are formed on a substrate 10c, is uniformly charged by corona charging with a charger 11, as shown exemplarily in FIG. 1(a). Then, image exposure is effected to form an electrostatic charge pattern corresponding to the image, as shown in FIG. 1(b). Thereafter, the photoconductive member is heated with a heater 12, with the electrode 10b grounded, as shown in FIG. 1(c). In consequence, the thermoplastic resin layer 10a is plasticized, and the electric surface charge and the electric charge of the opposite sign that is induced on the electrode 10b in correspondence to the electrostatic charge pattern attract each other. As a result, a dimple pattern image 10d, that is, a frost image, is formed on the surface of the thermo-plastic resin layer, as shown in FIG. 1(d). After the formation of the frost image, the photoconductive member is cooled to fix the dimple pattern image, thus enabling development of the electrostatic charge pattern.
However, the conventional developing method shown in FIGS. 1(a)-1(d) is inferior in the electric charge retaining performance because the electrostatic latent image is formed on the photoconductive member.
In addition, the recording medium that has a photoconductive layer needs an operation of effecting overall charging by, for example, corona charging, to perform recording.
Further, the conventional developing method shown in FIGS. 1(a)-1(d) cannot always obtain satisfactorily deep dimple patterns and hence cannot increase the density range.
In addition, the frost image cannot be formed unless the potential of the electrostatic latent image is higher than a predetermined level V1, and it is saturated at a voltage higher than a predetermined level V2. Thus, the tonal reproducibility is restricted within a predetermined potential range. In the meantime, the surface potential of an information recording medium shows characteristics relative to the exposure energy, such as those shown in FIGS. 2(a)-2(d). Thus, a frost image is formed within a surface potential range of from V1 to V2, and the tonal reproducibility is obtained only within this range. The range is determined by the kind of resin material used and the film thickness, and it is independent of the characteristics of the photosensitive member used. Accordingly, a frost image can be formed only within the range of exposure energy J that is defined as J1.ltoreq.J.ltoreq.J2. Thus, the tonal reproducibility is undesirably fixed in a narrow range.
When a frost image is formed from an electrostatic charge pattern that is formed on an information recording medium of high insulation quality and with a thermoplastic resin layer, this frost image can be visually observed by transmission or reflection of light. However, if the frost image is extremely fine, it is difficult to read it with high accuracy, and it has therefore been demanded to develop a reading method which makes full use of the high resolution of the frost image.
Incidentally, when an explanation of a material needs to be made to a large number of people, the use of an OHP is convenient and also advantageous in terms of cost because a troublesome operation must be conducted in order to prepare a number of copies of the material which corresponds to the number of people. An original for OHP has heretofore been made by reproducing an original document on an OHP film by toner development using a copying machine.
In the making of an OHP original by use of a copying machine that performs toner development, the resolution is limited by the particle size of the toner, so that it is difficult to make an original of high resolution. In addition, when the toner image on a photosensitive drum of the copying machine is transferred to an OHP film by thermal fusion, a resin material, which is used as a film material, causes sagging on heating, so that it has heretofore been difficult to make an OHP original of good quality.
Microfilms, transmission slidefilms and so forth are generally made by use of silver halide photography, diazonium salt electrophotography, toner development, etc. In any of these methods, however, the developing process is carried out in an off-line manner. In the case of toner development, wet toner must be employed and hence many problems are involved, for example, a complicated apparatus, maintenance, etc.
In the conventional frost image forming method, charging is effected by exposure under voltage application to form a potential pattern in accordance with the quantity of exposure energy, thus forming dimple patterns. Considering, for example, a case where light is applied to an original document to effect exposure by use of the reflected light therefrom, black portions of the original document absorb the light, whereas white portions of it reflect the light, so that no dimple patterns are formed on portions corresponding to the black portions of the document, but dimple patterns are formed on portions corresponding to the white portions, resulting in a negative image of the document. When it is desired to obtain a positive image, reversal development must be conducted.
It is an object of the present invention to provide an information recording medium which allows image formation by a simple method and which enables formation of a frost image of high quality and high resolution.
It is another object of the present invention to provide a frost image forming method which enables a frost image with relatively deep dimple patterns and which allows enlargement of the density range in development.
It is still another object of the present invention to provide a frost image forming method which enables tonal reproducibility to be selected as desired by varying .gamma.-characteristis of the surface potential relative to the quantity of exposure energy.
It is a further object of the present invention to provide a method of optically reading a frost image, in which scanning is effected with laser light and the transmitted or reflected light is read.
It is a still further object of the present invention to provide a method and an apparatus which can prepare an OHP original of good quality and high resolution without effecting toner development.
It is a still further object of the present invention to provide an apparatus for making an original for an overhead projector, a microfilm, a transmission slidefilm, etc., which can form a positive frost image directly.